The field of this invention is hydraulic cements, such as Portland cement. Heretofore, fly ash has been used as a cementing material in admixture with lime, or lime and soil, or similar mixture, but has not been converted to a hydraulic cement with properties similar to Portland cement.
It is known that phosphoric acid can be used for stabilization of soils, especially clay-type soils. See Demirel, et al., Proc. Iowa Academy of Sciences, 65, 311-316 (1958); and Highway Research Board Bulletin 318, "Soil Stabilization with Phosphorus Compounds and Additives," National Academy of Sciences--National Research Council, Washington, D.C. (1962). Demirel, et al. proposed that the cementing substance formed from the clay soils was a gel, and that since the formation of the gel must take place in solution, moist curing of the phosphoric acid treated soil was essential. Lyons, et al described the cementing substance as being formed from reaction of the phosphoric acid with the aluminosilicates of the clay. Highway Research Board Bulletin 318, page 13. Hemwall, et al. proposed that the cementing compound was a dihydroxy aluminum dihydrogen phosphate having the general formula Al(OH).sub.2 H.sub.2 PO.sub.4.xH.sub.2 0. Highway Research Board Bulletin 318, page 36. However, no generally accepted setting mechanism for the reaction of phosphoric acid with clay soils has been elucidated.